The purpose of this study is to determine a breath carbon monoxide (CO) cutoff level that optimally discriminates between heavy and light smokers and nonsmokers who are and who are not exposed to environmental tobacco smoke. Breath CO will be compared against cotinine concentration, which is the gold standard in verifying smoking status. The study will include four groups: 1) 55 smokers reporting >10 cigarettes per day, 2) 55 smokers reporting &#8804;10 cigarettes per day, 3) 55 nonsmokers reporting regular environmental exposure to tobacco smoke, and 4) 55 nonsmokers reporting limited or no environmental exposure to tobacco smoke (total of 110 smokers and 110 nonsmokers). Outcome measures include: 1) breath CO;2) semiquantitative salivary cotinine;3) quantitative salivary nicotine, cotinine, and 3-hydroxycotinine;4) semiquantitative urinary cotinine;5) quantitative urinary nicotine, cotinine, norcotinine, and 3-hydroxycotinine;and 6) self-reported smoking variables. During this past year, we have completed about 75% of subject testing. Because study enrollment is ongoing, we have no findings to report at this time.